The present invention relates to an aerogel composite blanket insulation product which is useful as a thermal protection system in environments which contain one or more solid conductive, gaseous convective, and/or radiative heat transfer components. In particular, the composite blanket insulation contains extremely low density aerogels formed amongst the interstices of a fibrous blanket matrix. The low density aerogels within the composite blanket insulation greatly improve the thermal conductivity performance of the fiber blanket insulation, i.e. reduces the thermal conductivity thereof. The aerogel composite insulation exhibits greatly improved thermal performance (lower thermal conductivity) by means of a combination of (a) reduced solid conduction by establishing a heat conduction path through the ultra-low thermal conductivity aerogels, (b) reduced gas molecular conduction through a suppresing gas convection utilizing the fine pore sizes of the aerogels, and (c) preferably certain modifications of the surface of the fiber materials to reduce infrared penetration through the fiber blanket.
Currently, highly evacuated multilayer insulation (MLI) and evacuated powder insulation are the most widely used insulation systems for cryogenic applications. Multilayer insulation is currently the most effective cryogenic insulation system when used under high vacuum. However, because of the highly anisotropic nature of MLI, its use on actual systems requires careful attention during installation and oftentimes causes awkward structural complexities. Furthermore, MLI is very expensive, bulky, heavy, and also requires the presence of a high vacuum state (10.sup.-4 Torr or better) to realize its full potential.
Evacuated aerogel powder insulation is approximately one order of magnitude less effective than MLI but is isotropic and generally easier to handle. It requires only a moderate vacuum (10.sup.-2 or 10.sup.-3 Torr). A major drawback to any powder insulation, however, is the tendency of a powder to settle over time and thereby form voids, particularly when the insulation is subject to vibration or thermal cycling. The settling can result in heat leakage in those areas where the voids have formed in the insulation.
Therefore, there has been a constant demand for cost effective easy-to-handle insulation systems, both for cryogenic and high temperature applications. Particularly desirable is an effective insulation suitable for use over irregular shaped components such as valves, pipe joints, penetrations, and the like. Recent advances have resulted in the development of ultra-low density aerogels. The highly porous structure of these aerogels results in a reduction in solid conductivity as compared to the powdered aerogels currently in use. While powdered aerogels can conform to the shape of any insulation space, they are limited in application and can be messy, especially when maintenance is required. Thus, while a powdered ultralow density aerogel can be tailored to meet the geometries of complex shapes, the products still suffer from the old problems of settling and void formation.
Aerogels have also been produced in monolith form. Although monolithic aerogels have been considered excellent candidates for window materials due to a relatively high thermal resistance and optical transparence, the monolithic materials have been rigid and fragile. The monolith aerogels are extremely difficult to handle and cannot easily be used to insulate complex shaped bodies.
Recent advances in fibrous materials has produced organic and inorganic fibers useful as thermal insulations over a wide range of temperatures. The fibrous insulations offer the benefits of flexibility, both at room and cryogenic temperatures and ease of installation. However, the thermal conductivity of these new materials has been only in the order of 0.005 W/m.degree. K. (R=28.8/inch) and 0.03 W/m.degree. K. (R=4.8/inch) for samples tested in vacuum and air, respectively. Since these fibrous products possess only a relatively moderate thermal conductivity, they exhibit limited insulating performance, particularly at extreme temperatures.
Several documents have disclosed the general formation of aerogels in the presence of fibers and other such materials. For example, U.S. Pat. No. 4,629,652 (Carlson et al) discloses forming pelletized aerogel products within a variety of support structures ranging from vermiculite to distillation rings. The aerogels cling tenaciously to the supporting structures and result in the production of "pellettized" aerogels. U.S. Pat. No. 5,306,555 (Ramamurthi et al) discloses preparing aerogel matrix monolith composites by mixing aerogel precursors with fibers, aging the fiber-containing aerogel precursor to obtain a gelled composition, supercritical drying the fiber-gel composition, and rapidly releasing the pressure from the supercritical temperature and pressure conditions. The resulting monolith structures are reported to have thermal conductivities of only about 0.018 to 0.021 W/m.degree. K. and thus R values of only about 6.8-8/inch in vacuum. The products of the present invention exhibit substantially superior performance characteristics for the same metal oxide.
The prior art fails to suggest impregnating a fibrous structure with an aerogel precursor such that the precursor forms a liquid phase around each fiber and then immediately supercritically drying the precursor solution to form aerogels distributed throughout the fibrous matrix such that essentially no fiber--fiber contacts remain (as determined by scanning electron microscopy examination). Moreover, the prior art does not suggest pre-treating a fiber matrix to enhance the attraction between the aerogels and the fibers of the matrix as well as to reduce thermal conduction through the resulting composite insulation in some cases. Furthermore, the maximum reported R value for low temperature aerogel powder products is about 140/inch and for low temperature aerogel monolithic products about 20/inch, both in high vacuums of less than about 10.sup.-5 Torr. Thus the prior art fails to suggest techniques for increasing these R values to the more than 300 obtained with the low temperature aerogel products of the present invention.
It has now been discovered that the performance of a fiber-reinforced aeroqel product may be improved by minimizing free gas molecular conduction and convection and by substantially precluding point contacts between fibers.
Accordingly, it is an object of the present invention to produce an aerogel-fiber composite product having superior insulation performance characteristics as compared to currently existing aerogel insulation materials or fiber blankets.
It is a further object to produce a flexible superinsulation composite product in an easy-to-use configuration by interstitially forming low thermal conductivity aerogels within a fibrous matrix.
It is a still further object to produce a flexible insulation product wherein there are no fiber--fiber contacts.
It is a still further object to enhance the interfacial attraction between aerogels and fibers.
It is a still further object to minimize infrared penetration through the composite insulation.
These and still other objects will be apparent from the following disclosure of the present invention.